West German Offenlegungsschrift (Unexamined Patent Application) No. 22 32 067 describes aqueous acidic phosphatising solutions, having a weight ratio of Zn:PO.sub.4 =1: (from 12 to 110), for the surface treatment of metals, particularly iron and steel. The reduced zinc content, in comparison with customary phosphatising baths, leads to improved thin and even phosphate coats, which have very good adhesion and stability and are eminently suitable as a foundation for the subsequent electrophoretic coating. It has proved very advantageous in this process to use nitrite in addition, for achieving the desired results.
West German Offenlegungsschrift No. 30 04 927 discloses a process for the phosphatising of metals that are subsequently electroplated, in which, similarly, acidic aqueous ainc phosphate solutions are employed. The solutions contain nitrite ions and/or aromatic nitrocompound as accelerators. The performance of the phosphatising operation takes place in a special dipping/spraying process.
The disadvantage of the known processes is that the preferably employed nitrite leads to the formation of nitrous gases, which, as is known, are toxic and malodorous. Extraction measures, therefore, are indispensable for ensuring works' safety.
It is, therefore, an object of the invention to provide a process, which does not possess the disadvantages of the known processes, requires little heating energy, owing to low application temperatures, and is easy to carry out.
This and other objects will become apparent to those in the art from the description of the invention which follows.